Enable custom role UI for SaaS

What does this MR do and why?

This is part 2 of 2. Part 1 is here.

This is the last step to enable the custom role UI for SaaS.

EE: true

Screenshots or screen recordings

How to set up and validate locally

1. Apply changes in !130089 (merged): curl https://gitlab.com/gitlab-org/gitlab/-/merge_requests/130089.diff | git apply
2. Follow the steps in !130089 (merged) (feature flag and license tier)
3. Go to group settings and select Roles and Permissions.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.